The present invention relates to a braided soft tissue prosthesis and, more particularly, to a soft tissue prosthesis formed from a three-dimensional braided structure.
Vascular grafts are commonly used as soft tissue prostheses to replace damaged or diseased veins and arteries. To maximize the effectiveness of any prostheses it is desirable that it have characteristics which closely resemble that of the natural body lumen.
One particular problem which is encountered is that of thrombosis. Thrombosis, or clotting, occurs when an individual's blood contacts a foreign body. As the blood begins to deposit platelets on the foreign body, a thrombus or blood clot forms. Historically, grafts having a relatively large diameter (greater than 10 mm) have generally proved successful over the long term because the build-up of thrombus that occurs on the interior surface of the graft is not sufficient to substantially obstruct the flow of blood. However, with respect to grafts having a diameter less than 10 mm, the build-up of thrombus on the interior surface of the graft can result in a complete obstruction of the graft in a relatively short period of time.
Presently, conventional tubular prostheses and, more specifically, vascular grafts formed by weaving or knitting synthetic fibers into a tubular structures, are susceptible to kinking or collapsing under varying circumstances, e.g., when the graft is bent during the contraction of surrounding muscle, or when external pressure is applied to the graft. One conventional solution to these problems has focused on the reinforcement of the walls of the vascular graft through the use of helically wrapped reinforcing fibers, reinforcing rings or bands placed externally around the tubular structure. The additional reinforcement of the tubular structure generally has the disadvantage of reducing the radial and/or longitudinal compliance of the graft due to the stiffness of the reinforcing member. A non-compliant graft may reduce the blood flow through the graft, thereby compromising the ability of the prosthesis to perform naturally. Additionally, the reinforcing member generally cannot be penetrated by cellular ingrowth from surrounding tissue and may cause the erosion of the surrounding tissue during contraction.
Another important characteristic associated with soft tissue prostheses is that of porosity. Preferably, the exterior surface of the prosthesis should include pores which are large enough to allow for the entry of connective tissue into the outer periphery of the graft. Conversely, the inner surface of the prosthesis must have pores small enough so that the blood or body fluid passing through the prosthesis will not leak into the prosthesis. Smaller pores on the inner surface of a vascular prosthesis also result in reduced platelet adhesion and a decreased amount of thrombus formation at the inner surface. Typically, a vascular prosthesis having a constant pore size throughout the structure requires pre-clotting in order to avoid leakage through the pores of the prosthesis; however, pre-clotting tends to increase the risk of contamination of the prosthesis as well as create a risk for clots to break off and form emboli.
Conventional tubular maypole (single layer) braided prosthesis have been tried in the past. However, due to their shortcomings, such prostheses have never been commercialized. One of the greatest disadvantages of a conventional tubular maypole braided prosthesis is the scissoring action which occurs under conditions of blood flow. More specifically, as blood is pumped through the graft, the pressure within the graft increases and decreases concurrently with the pumping of the heart, causing the yarns forming the braid to scissor correspondingly with the expansion and contraction of the graft. This scissoring action by yarns of the conventional maypole braided grafts tends to shear tissue which is attempting to grow into the vascular graft, thereby hindering the natural healing process and assimilation of the graft into natural tissue. Contrary to such conventional structures, the present invention concerns structures which due to their three-dimensional character are dynamically more stable and less prone to scissoring.
Yet another disadvantage of presently available woven or conventional tubular maypole braided prostheses is that sutures easily pull out making it difficult to attach the prosthesis to the existing body lumen and to prevent leakage at this junction. Also, since tubular prostheses are typically formed from a synthetic yarn in the form of a tube, the ends of the tube tend to easily ravel. This is true for single layered prostheses in general. Once the ends ravel or fray, suturing to the existing body lumen becomes extremely difficult. These difficulties explain the reason that these single layered braids have not been commercialized.
Accordingly, it would be advantageous to provide a new and improved soft tissue prosthesis that overcomes the previously-described disadvantages associated with presently available prostheses. More specifically, it would be particularly desirable to have a prosthesis which has the following characteristics: controlled porosity; ravel and fray resistance; a radially self-supporting structure to prevent kinking and collapsing of the prosthesis; and longitudinal compliance for ease of implantation and sizing.